Electronic module with layer of adhesive and process for producing it

ABSTRACT

The present invention relates to an electronic module having a layer of adhesive between metallic surfaces of components of the module. The metallic surfaces are arranged facing one another. The adhesive of the layer of adhesive includes agglomerates of nanoparticles, which form paths, surrounded by an adhesive base composition, in the adhesive base composition. Furthermore, the invention relates to a process for producing the module.

CROSS REFERENCE TO RELATED APPLICATIONS

This Utility Patent Application claims the benefit of the filing date ofGerman Application No. DE 102 06 818.6, filed Feb. 18, 2002, andInternational Application No. PCT/DE03/00458, filed Feb. 14, 2003, bothof which are herein incorporated by reference.

BACKGROUND

The invention relates to an electronic module with layer of adhesive,and to a process for producing it.

Layers of adhesive in electronic modules are used for a very wide rangeof purposes. Electronic modules are known in which materials of asimilar type, such as semiconductor chips, are mechanically joined via alayer of adhesive and stacked, or metallic surfaces are provided withcorresponding metallic flat conductors, or wherein individual ceramiclayers are adhesively bonded to form a multilayer ceramic substrate.Moreover, electronic modules are known in which materials of differenttypes are joined to one another by means of a layer of adhesive, forexample a semiconductor chip is joined to a metallic chip island, or asemiconductor chip is joined to a ceramic substrate or a metallic flatconductor is joined to a ceramic substrate.

The thermal and electrical properties of the layer of adhesive can bevaried by admixing fillers into the layer of adhesive. Some electronicmodules have electrically conductive layers of adhesive, and someelectronic modules have electrically insulating layers of adhesive. Onedrawback of conventional layers of adhesive is that, despite havingparticulate fillers, they have completely isotropic properties withregard to the electrical conductivity and/or the thermal conductivity.On account of the isotropic electrical conductivity in an electricallyconductive layer of adhesive, not only are metallic surfaces facing oneanother electrically connected to one another and short-circuited, butalso electrically conductive surfaces arranged next to one another areshort-circuited. The same drawback arises with layers of adhesive withan isotropic thermal conductivity, in which the thermal energy cannot bedeliberately dissipated in one direction, but rather is transmitteduniformly in all directions.

A further drawback of filled layers of adhesive is their minimumthickness. Whereas the adhesive base composition of a layer of adhesivecan be made as thin as desired, depending on the viscosity, with filledlayers of adhesive which are intended to improve either the thermalproperties or the electrical properties of the adhesive basecomposition, a thickness of 5 micrometers or more is required in orderfor the filler to be incorporated in a sufficient thickness andconcentration in the adhesive base composition.

A further drawback of such thick filled layers of adhesive is that ameniscus is formed in the edge region, requiring increased areadepending on the wetting properties and the adhesive thickness.

SUMMARY

One embodiment of the present invention provides an electronic modulehaving a layer of adhesive and a process for producing it in which theabove drawbacks are overcome and it is possible to reduce the spacerequired and to improve the reliability of the layer of adhesive.

According to one embodiment of the invention, the electronic module hasan electrically conductive layer of adhesive between metallic surfacesof components of the module. The metallic surfaces are in this casearranged facing one another. The adhesive of the electrically conductivelayer of adhesive includes agglomerates of electrically conductivenanoparticles. These agglomerates are surrounded by an adhesive basecomposition and form electrically conductive paths in the adhesive basecomposition. The surfaces that face one another are electricallyconnected to one another in a punctiform manner via a multiplicity ofagglomerates of electrically conductive nanoparticles which are randomlydistributed in the polymer layer.

The formation of agglomerates of electrically conductive nanoparticlesis promoted by the favorable ratio between the surface area of thenanoparticles and the volume of the nanoparticles. This property isachieved by virtue of the fact that the minimal dimensions of theparticles, in the nanometer range, improve their mobility in theadhesive base composition. Moreover, their infinity for one another isgreatly increased on account of the greater ratio between surface areaand volume for nanoparticles compared to microparticles. Secondly, theseagglomerates of nanoparticles in the adhesive base composition have theadvantage that they can adapt themselves to the distance betweenopposite surfaces. A further advantage of a layer of adhesive of thistype having agglomerates of electrically conductive nanoparticles is theanisotropy of the electrical conductivity in layers of adhesive.

Since each agglomerate is surrounded by an insulating adhesive basecomposition, isolated electrically conductive paths, which connect theelectrically conductive surfaces that face one another to one another ina punctiform manner, result at the positions of the agglomerates.Electrically conductive surfaces which are arranged next to one another,however, are not short-circuited with one another on account of theisotropy of the layer of adhesive. Therefore, the layer of adhesive cansuccessfully connect without any short-circuiting a plurality ofmetallic surfaces, arranged next to one another, of microscopicallysmall dimensions, to metallic surfaces of corresponding dimensionsarranged facing them. In this context, the term microscopically small isto be understood as meaning a dimension which can only be detected andmeasured under an optical microscope.

The electrically conductive nanoparticles include gold, silver, copper,nickel or alloys thereof. These nanoparticles of the corresponding noblemetals and/or also of copper and its alloys have the advantage of beingparticularly successfully adsorbed, and consequently with a size in theorder of magnitude of nanoparticles they tend to form agglomerates in anadhesive base composition. Furthermore, the metals gold, silver andcopper have a high electrical conductivity.

On account of the electrically conductive nanoparticles, the thicknessof the layer of adhesive may be less than one micrometer. Such a thinlayer of adhesive not only improves the space requirement but alsominimizes the area requirement, since a correspondingly small meniscuscan form. At the same time, the electrical conductivity of the layer ofadhesive is improved by a factor of over five compared to layers ofadhesive containing electrically conductive microparticles.

Components with different coefficients of thermal expansion may differin terms of their expansion coefficient by a factor of up to threewithout the risk of damage occurring in the event of fluctuating thermalloading of the electronic component, provided that the thickness of thelayer of adhesive is a multiple of the mean diameter of the electricallyconductive nanoparticles.

The nanoparticles may form between 30 and 95% by weight. This level ofnanoparticles corresponds to a filling level of between approximately 4and 70% by volume. This means at least 30% of the volume of the layer ofadhesive consists of the insulating adhesive base composition, whichensures that the agglomerates of electrically conductive metallicnanoparticles are completely surrounded by the insulating adhesive basecomposition. In the case of layers of adhesive in electronic modules, ananisotropically electrically conductive structure is formed, with aninsulating action in the plane of the layer of adhesive and anelectrically conductive action orthogonally with respect to the layer ofadhesive, i.e. over the thickness of the layer of adhesive.

In the electrically conductive layer of adhesive with electricallyconductive nanoparticles, the agglomerates of nanoparticles in theadhesive base composition are deformable. On account of thisdeformability, the length of the conductive paths can be adapted to thecorresponding thickness of the layer of adhesive, so that it is possibleto compensate for differences in the distance between the electricallyconductive surfaces facing one another. In particular in the case ofsemiconductor chips, the height or depth of the electrically conductivecontact surfaces on the surface of the semiconductor chips is subject toconsiderable stagger, which means that this property of the electricallyconductive agglomerates of nanoparticles being deformable isparticularly suitable for connecting contact surfaces on the active topsurfaces of the semiconductor chips to one another in an electricalmodule comprising stacked semiconductor chips.

The nanoparticles may have a mean diameter of between 10 and 200nanometers. A mean diameter of from 10 to 50 nanometers is particularlysuitable for extremely thin layers of adhesive with a thickness of muchless than one micrometer, while the upper range of 100 to 200 nanometersis intended for layers of adhesive with thicknesses of approximately onemicrometer. The use of electrically conductive nanoparticles with a meandiameter within these ranges between 10 and 200 nanometers thereforeallows the electrically conductive layer of adhesive to be veryaccurately matched to the possible distance between the surfaces whichface one another and are to be electrically connected to one another.

The mean diameter of the agglomerates of electrically conductivenanoparticles in the adhesive may be as much as the thickness of thelayer of adhesive. This ensures that a reliable connection via suitableconduction paths which are formed from deformed agglomerates ofelectrically conductive nanoparticles in the layer of adhesive isproduced in every region of the layer of adhesive.

As starting material, the adhesive may include a polyamide acetate whichis enriched with electrically conductive nanoparticles and is dissolvedin N-methylpyrrolidone. This starting material is such that theviscosity of the polyamide acetate dissolved in N-methylpyrrolidone canbe varied by means of the proportion of N-methylpyrrolidone and canthereby be matched to the size of the nanoparticles.

In addition to the nanoparticles, the adhesive includes catalystmaterials and adhesion promoters in an adhesive base composition formedfrom polyamide. In this context, polyamide is not adhesive on its own,and consequently in particular the addition of an adhesion promoter isresponsible for the adhesive action of the polyamide base composition.Catalyst materials in this adhesive base composition are intended tohelp accelerate the crosslinking of the polyamide. Therefore, theadditions of catalyst materials and adhesion promoter in combinationwith the electrically conductive nanoparticles result in an adhesivewhich has an improved adhesion and a higher crosslinking rate combinedwith a reduced crosslinking temperature.

At least one of the metallic surfaces facing one another may be arrangedon a semiconductor chip. This enables the semiconductor chip and itsmetallic surfaces to be connected to metallic surfaces, for example ofceramic substrates, or directly to metallic chip islands of a leadframeusing this adhesive.

Furthermore, at least one of the metallic surfaces which face oneanother and between which an electrically conductive layer of adhesiveis to be arranged may be arranged on a ceramic substrate. Electronicmodules in which at least one of the metallic surfaces has a ceramicsubstrate have the advantage over electronic modules in which, forexample, metallic surfaces are arranged on a glass fiber-reinforcedcircuit board material that the coefficient of thermal expansion of aceramic substrate is much closer to the expansion coefficient of asemiconductor chip. This allows the layer of adhesive to be formed witha thickness of less than one micrometer without delamination occurringas a result of thermal stresses.

The smallest thickness of a layer of adhesive can be used if the twocomponents of an electronic module which are to be adhesively bonded toone another and electrically connected to one another consist ofidentical materials, since the layer of adhesive does not in any wayhave to form a buffer for the thermal expansion properties of thematerials. A layer of adhesive comprising agglomerates of electricallyconductive nanoparticles is therefore particularly suitable for use instacks of semiconductor chips. A layer of adhesive of this type can alsobe used for stacks of individual ceramic layers which are intended toform a multilayer ceramic substrate. Therefore, by stacking materials ofthe same type and joining them using an adhesive comprising agglomeratesof electrically conductive nanoparticles, it is possible to realizeextremely compact, space-saving electronic modules.

A process for producing an electronic module having an electricallyconductive layer of adhesive between metallic surfaces, which face oneanother, of components of the module includes the following processsteps. First of all, a starting solution is produced by dissolving apolyamide acetate in N-methylpyrrolidone. Then, this starting solutionis mixed with catalyst materials and adhesion promoters to form anadhesive solution. This adhesive solution is mixed with electricallyconductive nanoparticles to form an electrically conductive adhesive.Then, the adhesive can be applied to at least one of the top surfaces ofthe components of the electronic module which are to be adhesivelybonded, and then the components are joined together.

Finally, the adhesive crosslinks with heating and mechanically connectsthe components and, if the components have metallic surfaces facing oneanother, these metallic surfaces are connected to one another by theagglomerates of electrically conductive nanoparticles which form in theadhesive. With this process, it is possible to produce a layer ofadhesive which has anisotropic properties in terms of the electricalconductivity. On account of the high specific surface area of theelectrically conductive nanoparticles with respect to their volume, ahigh affinity is produced between the electrically conductivenanoparticles even during the mixing of the adhesive solution withelectrically conductive nanoparticles, with the nanoparticles joiningtogether to form agglomerates, the agglomerates being surrounded by theinsulating adhesive solution on all sides.

When the adhesive is applied to at least one of the top surfaces of thecomponents which are to be adhesively bonded in a thickness which isless than the mean diameter of the agglomerates, a layer of adhesive isprepared which electrically connects metallic surfaces facing oneanother, whereas metallic surfaces of the individual components locatednext to one another remain isolated from one another.

The temperature for crosslinking the adhesive is over 100° C., so thatthe components are heated to a temperature of this nature in order tocrosslink the adhesive to form a secure bond. The crosslinking time andthe crosslinking temperature can be reduced by the catalyst materialscontained in the adhesive solution.

The metallic surfaces of the components which are to be electricallyconnected are oriented with respect to one another before the componentsare joined together. This step is made easier by the fact that theadhesive only has to be applied to one of the two top surfaces of thecomponents which are to be adhesively bonded. During this application,the agglomerates of electrically conductive nanoparticles of gold,silver, copper, nickel or alloys thereof are arranged isolated from oneanother and in a random distribution in the layer of adhesive betweenthose surfaces of the components which are to be electrically connectedand have been oriented, prior to crosslinking.

The agglomeration of the electrically conductive nanoparticles allows aplurality of metallic surfaces which are arranged next to one anotherand are of microscopically small dimensions to be electricallyconnected, without any short-circuiting, to metallic surfaces, arrangedfacing them, of the components as soon as the two components have beenjoined together. If the components comprise semiconductor chips, thesechips can be connected to form a stack of electrically conductivesemiconductor chips. A stack of this type has a reduced overall height,especially since the agglomerated electrically conductive nanoparticles,depending on the viscosity of the adhesive base composition and the meandiameter of the electrically conductive nanoparticles, may form a layerof adhesive with a thickness of less than one micrometer.

A first embodiment of the invention realizes very low electricalresistances combined with high electrical conductivities whilesubstituting solders if, in the context of this first embodiment of theinvention, an electrically conductive adhesive comprising agglomeratesof electrically conductive nanoparticles is used. The adhesive materialcan in this case be used to a limited extent as a buffer for differingthermomechanical expansions. The lower the difference in the coefficientof thermal expansion between the components of an electronic module, themore reliably it becomes possible for the components to be secured toone another with the aid of the adhesive according to the invention.This applies in particular to chip-on-chip connections if an electricalconnection between the two chips is required. Furthermore, metallicsurfaces as semiconductor chip islands on a ceramic substrate withsemiconductor chips can be covered, and at the same time an electricallyconductive layer of adhesive can be achieved.

A further embodiment of the invention relates to an electronic modulehaving a thermally conductive and electrically insulating layer ofadhesive between surfaces of components of the module. For this purpose,the surfaces are arranged facing one another. The adhesive of this layerof adhesive includes agglomerates of thermally conductive, electricallyinsulating nanoparticles. These agglomerates are surrounded by anadhesive base composition in which thermally conductive paths arearranged. These thermally conductive paths form thermal connections in apunctiform manner for the surfaces facing one another, with amultiplicity of these agglomerates being randomly distributed in thelayer of adhesive.

A module of this type is such that when dissipating heat lost from theelectronic components included in the electronic module, it candissipate the heat in a direction which is predetermined by the layersof adhesive. This direction for heat dissipation in an electronic modulecan start from the active top surface of a semiconductor chip and berouted via corresponding metallic cooling surfaces to one of the outersides of the electronic module. In this case, the semiconductor chip issecured to the metallic cooling surface with the aid of an electricallyconductive and thermally insulating layer of adhesive.

Furthermore, it is possible for a plurality of metallic surfaces thatare arranged next to one another and are of microscopically smalldimensions to be thermally conductively connected, without anyshort-circuiting, to metallic surfaces, arranged facing them, of thecomponents. In this way, even highly loaded interconnect structures canbe thermally conductively connected to a heat sink of the electronicmodule via the layer of adhesive without short circuits being causedbetween the highly loaded interconnects by the metallic heat sink.

The intensity of the cooling provided by the heat sink of an electronicmodule of this type is oriented and improved with the aid of thethermally conductive and electrically insulating paths which occur in apunctiform manner.

In one embodiment, the electronic module includes, as thermallyconductive and electrically insulating nanoparticles, particles ofsilicon dioxide, aluminum nitride, boron nitride,polytetrafluoroethylene or mixtures thereof. These nanoparticles havethe advantage that, on account of their high specific surface area inrelation to the particle volume, they combine with one another to formthermally conductive agglomerates with respect to the surroundingadhesive base composition. At least in the case of silicon dioxide,aluminum nitride and boron nitride, these agglomerates comprisethermally conductive ceramic particles which as their unit cell have ineach case just two types of atoms and therefore have a high thermalcoupling capacity. The improvement to the thermal conductivity of theadhesive base composition produced by polytetrafluoroethylene issubstantially based on the crystalline nature of the tetrafluoroethylenecomprising polymer products.

In one embodiment, the layer of adhesive, including its electricallyinsulating and thermally conductive nanoparticles, has a thickness ofless than one micrometer. Such thin layers of adhesive cannot beachieved with microparticles with a mean diameter in the micrometerrange. Furthermore, the ability to form agglomerates between particleson the micrometer scale is extremely low compared to nanoparticles, onaccount of the increased volume compared to the surface area.

With such thin layers of adhesive, the buffering compensation betweenthe components consisting of materials with different coefficients ofthermal expansion is limited, and consequently the coefficients ofthermal expansion of the components may only differ by at most a factorof three. The smaller the difference in the coefficient of thermalexpansion between the components, the thinner it is possible for a layerof adhesive comprising agglomerates of thermally conductive andelectrically insulating nanoparticles to be.

In one embodiment, the electrically insulating and thermally conductivenanoparticles may form between 30 and 90% by weight of the layer ofadhesive. Based on the abovementioned thermally conductive butelectrically insulating materials with a relative density of between 2and 3.3, the thermally conductive nanoparticles then produce between 4and 70% by volume. Therefore, if the nanoparticles form between 30 and95% by weight, a nonconductive, thermally insulating volume of adhesivebase composition of from 30 to 95% by volume remains in the layer ofadhesive, surrounding the individually thermally conductive paths formedfrom agglomerates of thermally conductive and electrically insulatingnanoparticles. Therefore, in the case of layers of adhesive with athickness of less than one micrometer, the result is a layer of adhesivewith an anisotropic thermal conductivity.

In one embodiment, the thermally conductive and electrically insulatingnanoparticles have a mean diameter of between 10 and 200 nanometers.Nanoparticle diameters of between 10 and 50 nanometers can be used forlow-viscosity adhesive base compositions with a viscosity similar tothat of water, whereas nanoparticles with a mean diameter of between 50and 200 nanometers can be used successfully for thicker layers ofadhesive which, at the same time, have a higher viscosity.

To produce a reliable, thermally conductive contact with theagglomerates of thermally conductive, electrically insulatingnanoparticles in an adhesive or in a layer of adhesive, the meandiameter of the agglomerates is greater than the thickness of the layerof adhesive. This simultaneously ensures that each of the agglomerateswhich form contributes to the thermal conductivity between oppositesurfaces. At the same time, these agglomerates have the property ofbeing readily deformable such that they adapt to the distance betweenthe two opposite surfaces which are to be thermally connected.

In addition to the thermally conductive and electrically insulatingnanoparticles, an adhesive base composition also includes adhesivecatalyst particles and adhesion promoters if the adhesive basecomposition used is a polyamide. Polyamides do not naturally haveadhesive properties, and consequently an adhesive can only be formed bythe adhesion promoter. The catalyst particles which are present inaddition to the thermally conductive and electrically insulatingnanoparticles serve to accelerate the crosslinking of the layer ofadhesive to form a polyamide and to reduce the crosslinking temperature.

In one embodiment, the surfaces that face one another may be arranged onsemiconductor chips if the electronic module includes a stack ofsemiconductor chips. However, it is also possible for at least one ofthe surfaces which face one another to belong to a semiconductor chip,while the other of the surfaces belongs to a ceramic substrate or aglass fiber-reinforced circuit board. In this case, it is possible forceramic substrates to be connected to the semiconductor chip using verythin layers of adhesive with a thickness of less than one micrometer,especially since the difference in the expansion coefficient does notexceed a factor of three. However, problems arise with circuit boardmaterials, since some circuit board materials far exceed the factor ofthree with regard to the expansion coefficient. In this case, there is arisk of delamination of the semiconductor chip from the circuit board.

A process for producing an electronic module having a thermallyconductive layer of adhesive between surfaces, which face one another,of components of the module includes the following process steps. Firstof all, a starting solution is produced by dissolving a polyamideacetate in N-methylpyrrolidone. This starting solution is mixed with acatalyst material and with an adhesion promoter to form an adhesivesolution. Thermally conductive and electrically insulating nanoparticlesare then admixed to this adhesive solution. During this admixing, thehigh surface area to volume ratio of the nanoparticles results in theformation of agglomerates of electrically insulating and thermallyconductive nanoparticles. The adhesive can then be applied to at leastone of the top surfaces of the components of the electronic module whichare to be adhesively bonded to one another. After the components havebeen joined together, the adhesive will then crosslink with heating ofthe components.

This process produces electronic modules that have components that arethermally connected to one another but electrically insulated from oneanother. The thermal connection is in this case effected via thermallyconductive paths in the layers of adhesive connecting the components. Ananisotropy in the thermal conductivity of this nature has the advantagethat the heat can be deliberately dissipated into a direction from aheat-generating element in the electronic module toward the outer sideof the electronic module and, for example, to a cooling plate withoutelectrical short-circuiting occurring.

Crosslinking of the adhesive occurs at temperatures over 100° C.; thislow temperature allows that the individual components of the module arenot damaged by the action of heat.

If certain components are to be connected to one another in anelectrically insulated but thermally conductive manner, the surfaces ofthe components are oriented toward one another before they are joinedtogether.

Only after the surfaces which are to be thermally connected to oneanother have been accurately oriented with respect to one another canthe adhesive crosslinking take place at the corresponding crosslinkingtemperature.

The electrically insulating and thermally conductive nanoparticles haveagglomerated even before the crosslinking of the adhesive, so that whenthe surfaces which are to be adhesively bonded and thermally connectedto one another are brought together, these agglomerates can be deformed,so that it is possible to produce thermally conductive paths between thelocations which are to be connected. Prior to the crosslinking, theseagglomerates arrange themselves in the polymer layer between thesurfaces of the components which are to be thermally coupled in a formin which they are isolated from one another and distributed randomly,the thermally conductive nanoparticles substantially including silicondioxide, aluminum nitride, boron nitride, polytetrafluoroethylene ormixtures thereof. The agglomerates can still be deformed and adapt tothe distance between the surfaces facing one another. The agglomeratesof thermally conductive and electrically insulating nanoparticles remainisolated from one another and therefore connect a plurality of metallicsurfaces arranged facing one another without forming thermal bridges toadjacent surfaces.

One embodiment of the invention results in very low thermal resistances,since the adhesive bonds can be effected with very small layerthicknesses and just a low thermal resistance. In this case too, it is acondition that the components which are to be adhesively bonded, assubstrates or as semiconductor chips, must not have excessivelydivergent coefficients of thermal expansion. The adhesive material forsuch a thin layer of adhesive has only a reduced buffer action fordiffering thermomechanical expansions. In this case too, the adhesiveaccording to one embodiment of the invention and a layer of adhesiveaccording to one embodiment of the invention can be used forchip-on-chip applications if an electrically insulating connectionbetween the two chips is desired, and it is also possible for chips tobe secured to semiconductor chip islands of a ceramic substrate, sincein this case the differences between the coefficients of thermalexpansion can be buffered by the thickness of the layer of adhesive. Itis more difficult to adhesively bond materials with very differentcoefficients of thermal expansion, such as for example to adhesivelybond a silicon semiconductor chip, at 3 ppm/K, to a circuit board, whichhas a coefficient of thermal expansion of between 15 and 30 ppm/K. Ingeneral, the reduced layer thickness compared to layers of adhesive withparticles on a micrometer scale as filler reduces the materials costs byreducing the quantity of adhesive employed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of this specification. The drawings illustrate theembodiments of the present invention and together with the descriptionserve to explain the principles of the invention. Other embodiments ofthe present invention and many of the intended advantages of the presentinvention will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 illustrates a diagrammatic cross section through an electronicmodule of a first embodiment of the invention.

FIG. 2 illustrates a diagrammatic cross section through an electronicmodule of a second embodiment of the invention.

FIG. 3 illustrates a diagrammatic cross section through an electronicmodule of a third embodiment of the invention which includes a stack oftwo semiconductor chips.

FIG. 4 illustrates a diagrammatic cross section through an electronicmodule of a fourth embodiment of the invention which includes a stack ofa plurality of semiconductor chips.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments of the present invention can be positioned ina number of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

FIG. 1 illustrates a diagrammatic cross section through an electronicmodule 1 of a first embodiment of the invention.

Reference numeral 2 denotes a layer of adhesive which connects metallicsurfaces 3 of two components 4 of the electronic module 1 to oneanother. Reference numeral 5 denotes an adhesive of the layer ofadhesive 2, and reference numeral 6 denotes agglomerates which includenanoparticles 7. The agglomerates 6 are surrounded by an adhesive basecomposition in the adhesive 5 of the layer of adhesive 2. Paths whoseelectrical and thermal properties differ from the electrical and thermalproperties of the surrounding adhesive base composition 8 extend throughthe layer of adhesive 2 within the agglomerates 6. Reference numeral 10denotes one of the components 4 of the electronic module 1, in the formof a semiconductor chip. Reference numeral 11 denotes a ceramicsubstrate as second component 4, a metallic surface 3 of which isarranged facing the metallic surface 3 of the semiconductor chip 10.

On the top surface 13 facing the semiconductor chip, the ceramic plate11 has a rewiring level 14, the rewiring lines 15 of which areconnected, via bonding wires 16, to contact surfaces 17 on the activetop surface 18 of the semiconductor chip 10. The back surface 19 of thesemiconductor chip 10 is metalized and has the metallic surface 3. Asemiconductor chip island 20, which is connected via a through-contact21 to a rewiring level 24 on the underside 28 of the ceramic substrate11, is arranged on the ceramic substrate 11 facing this metallic surface3. The rewiring level 24 on the underside 28 of the ceramic substrate 11includes, in addition to rewiring lines 15, external contact surfaces 30on which external contacts 31 are arranged.

In this first embodiment of the invention, the external contacts 31 areballs of solder which protrude from the underside 28 of the ceramicsubstrate and are separated from one another by a soldering stop layer32. To electrically connect the back surface of the semiconductor chip10 to one of the external contacts 31, the layer of adhesive 2 is filledwith electrically conductive nanoparticles 7. For one embodiment of theinvention, the nanoparticles consist of electrically conductivematerial, such as gold, silver, copper, nickel or mixtures thereof.These electrically conductive nanoparticles, which take up to 70% byvolume of the layer of adhesive, agglomerate, in the uncrosslinked stateof the adhesive base composition 8 surrounding them, to formagglomerates 6 which are deformable in the uncrosslinked state of theadhesive 5 of the layer of adhesive 2. At the same time, theseagglomerates 6 of electrically conductive nanoparticles 7, in oneembodiment of the invention, form electrically conductive paths 9, sothat the layer of adhesive acts anisotropically as an electricalconnection between the metallic surfaces 3 facing one another. Whereasthe layer of adhesive 2 has an insulating action in the horizontaldirection, it connects the back surface of the semiconductor chip 10 tothe metallic chip islands 20 of the ceramic substrate 11 across thelayer thickness d of the layer of adhesive 2.

In another embodiment of the invention, this electrical connection is tobe interrupted, and only a thermal connection to the chip island 20 isto be used in order to dissipate heat via the chip island 20 to thethrough-contact 21 and the external contact 31. In one embodiment, thenanoparticles 27 likewise form agglomerates 26, but the nanoparticles 27are composed of electrically insulating but thermally conductivematerials, such as silicon dioxide, boron nitride andpolytetrafluoroethylene or mixtures thereof.

The nanoparticles 7 or 27 allow the thickness d of the layer of adhesiveto be minimized to less than one micrometer. The mean diameter of thenanoparticles 7 or 27 is between 10 and 200 nanometers, while the meandiameter of an agglomerate 6 or 26 of nanoparticles 7 or 27 is greaterthan the layer thickness d. This produces a multiplicity of connectionpoints, which are distributed randomly in the layer of adhesive 2,between the surfaces 3 or 23 facing one another. The deformability ofthe agglomerates 6 or 26 is utilized during production of themultiplicity of contact points, provided that the adhesive basecomposition has not yet been crosslinked. To crosslink the adhesive basecomposition, the components 10 and 11 are heated to a crosslinkingtemperature of over 100° C.

During production of an electronic module 1 of this type, first of all aceramic substrate 11 having the rewiring levels 15 on the top surface 13and the underside 28 and having through-contacts 21 is produced. In theregion of the semiconductor chip 10 which is to be attached, asemiconductor chip island 20 is provided on the rewiring level 15, andan adhesive 5 filled with nanoparticles is applied to this semiconductorchip island 20. The adhesive 5 itself is produced form a startingsolution in which a polyamide acetate is dissolved inN-methylpyrrolidone. Then, this starting solution is mixed with catalystmaterials and adhesion promoters to form an adhesive solution. Finally,either electrically conductive or electrically insulating nanoparticles7 or 27, respectively, are admixed to this adhesive solution. Thesenanoparticles 7 or 27 agglomerate to form larger agglomerates 6 or 26,respectively, the mean diameter of which is greater than the thicknessof the layer of adhesive 2 which is to be produced.

Then, this adhesive 5 is applied to the semiconductor chip island 20,the semiconductor chip 10 is put in place and the ceramic substrate 11together with the semiconductor chip 10 is exposed to a crosslinkingtemperature of over 100° C. After crosslinking of the adhesive 5, thebonding wire 16 can be bonded to the contact surfaces 17 on the activetop surface 18 of the semiconductor chip 10 and connected to therewiring lines 14 on the top surface 13 of the ceramic substrate 11.Then, a polymer potting compound 33 is applied to the top surface 13 ofthe ceramic substrate 11 so as to simultaneously encapsulate thesemiconductor chip 10, the layer of adhesive 2 and the bonding wires 16.Finally, external contacts 31 are applied to the external contactsurfaces 30 provided for them on the underside 28 of the ceramicsubstrate 11.

The soldering stop layer 32 may either be applied directly duringproduction of the ceramic substrate 11 or may be applied to theunderside 28 of the ceramic substrate 11 before the external contacts 31are put in place, leaving clear the external contact surfaces 30. Withthe embodiment illustrated in FIG. 1, it is optionally possible for thesemiconductor chip 10 either to be connected to the external contact 31in an electrically insulating and thermal manner or to be connected tothe external contact in an electrically conductive manner. For thispurpose, it is merely necessary to select a suitable material for thenanoparticles 7 or 27 in the layer of adhesive 2.

FIG. 2 illustrates a diagrammatic cross section through an electronicmodule 1 of a second embodiment of the invention. Components having thesame functions as in FIG. 1 are denoted by the same reference numeralsand are not explained once again.

In this embodiment of the invention, the components 4 of the electronicmodule 1 are formed from a semiconductor chip 10 and a metallic chipisland 20. The metallic chip island 20 simultaneously forms an outerside of the housing 34 of the semiconductor chip. The semiconductor chipisland 20 may either serve as a ground contact, in which case the layerof adhesive 2 will include agglomerates 6 which comprise electricallyconductive nanoparticles 7. However, if the back surface 19 of thesemiconductor chip 10 is only to be thermally connected to a heat sinkvia the semiconductor chip island 20, while at the same time beingelectrically insulated from this heat sink, thermally conductive butelectrically insulating nanoparticles 27, for example formed fromsilicon dioxide, boron nitride, aluminum nitride,polytetrafluoroethylene or mixtures thereof, are used for the layer ofadhesive 2. The agglomeration of the nanoparticles 27 ensures that thethermal conductivity of the layer of adhesive 2 is anisotropic and isoriented in the direction toward the chip island 20 and therefore towardthe heat sink.

To produce an electronic module 1 of this type, first of all the chipisland 20 and the metalization cap provided for the outer contacts 31are deposited by electroplating on a metal support (not shown). Then,the surface of the cap intended for the external contacts 31 is providedwith a bondable coating 35. A bondable coating 35 of this type for itspart includes a plurality of individual layers, namely a layer ofnickel, which inhibits copper diffusion, directly on the cap for theexternal contact 30 made from copper, and then a noble metal layer onthe nickel layer, for reliable bonding. Structuring of this nature hasthe advantage of preventing diffusion of copper ions into the connectionto the bonding wire 16 and thereby suppressing premature embrittlementof the bonded join. It is then possible for the adhesive 5 to be appliedto the chip island 20 in a thickness which is less than the meandiameter of the agglomerates of nanoparticles.

After the semiconductor chip 10 has been applied to the layer ofadhesive 22 and the adhesive base composition has been crosslinked at acrosslinking temperature of over 100° C., the bonding wires 16 areapplied to the contact surfaces 17 on the active top surface 18 of thesemiconductor chip 10 and are connected to the bondable coating 35.Finally, the entire metal support (not shown) is covered with a polymerpotting compound 33 which embeds both the layer of adhesive 2 and thesemiconductor chip 10 and the bonding wires 16. The metal carrier (notshown) can then be etched away, producing the exemplary embodiment whichis illustrated here in cross section. The adhesive 5 is produced in thesame way as in the first exemplary embodiment, and consequently there isno need to list the production steps.

FIG. 3 illustrates a diagrammatic cross section through an electronicmodule 1 of a third embodiment of the invention, which includes a stack12 of two semiconductor chips 10. Components which have identicalfunctions to the previous figures are denoted by the same referencenumerals and are not explained once again.

On their active top surface 18, the two semiconductor chips 10 havecontact surfaces 17, and on this top surface they bear an insulatingrewiring layer 36. Rewiring lines 15 are arranged on the insulatingrewiring layer 36. These rewiring lines connect the contact surfaces 17to metallic surfaces 3 on the insulating rewiring layer 36. The metallicsurfaces 3 on the two semiconductor chips 10 are oriented and arrangedin such a manner that they face one another. The bonding channels 37 ofthe two semiconductor chips 10, which have the contact surfaces 17, arearranged offset with respect to one another and are covered by a polymerpotting compound 33. In the remaining region between the twosemiconductor chips 10 there is arranged a layer of adhesive 2 formedfrom an adhesive 5 which has an anisotropic electrical conductivity.This anisotropic electrical conductivity is achieved by virtue ofagglomerates 6 of electrically conductive nanoparticles 7 being arrangedin an adhesive base composition 8.

The electrically conductive agglomerates 6 are isolated from one anotherby the adhesive base composition 8, so that the individual externalcontact surfaces 30, which face one another, are electrically connectedvia paths 9 through the agglomerates 6. However, there is no electricalconnection between external contacts 30 located next to one another.Consequently, the adhesive 5 can be applied over a large area and inlayers without having to selectively concentrate on the metallicsurfaces 3 facing one another. Rather, the selectivity is effectedautomatically by the anisotropy of the electrically conductive layer ofadhesive. Despite uniform application of the adhesive 5, the twosemiconductor chips are connected by means of their correspondingexternal contact surfaces 30 and therefore by means of their contactsurfaces 17 via the rewiring lines 15. As in the first embodiment, asoldering stop resist layer 32 between the external contact surfaces 30can in each case ensure that the rewiring lines 14 remain insulated andthat only corresponding external contact surfaces 30 are connected toone another via the electrically conductive agglomerates.

FIG. 4 illustrates a diagrammatic cross section through an electronicmodule 1 of a fourth embodiment of the invention, which includes a stack12 of a plurality of semiconductor chips 10. Components bearing the samereference numerals as in the previous figures are denoted by the samereference numerals and are not explained once again.

In this embodiment of the invention, six similar components 4 of anelectronic module 1 in the form of semiconductor chip diodes are stackedon top of one another. For this purpose, both the n-conducting cathode38 and the p-conducting anode 39 are metalized on their respective outersides, and then, for a cascade circuit of diodes of this type, therespective anode 39 is electrically connected to the cathode 38 of thenext diode up by means of an electrically conductive layer of adhesive2. For this purpose, the layer of adhesive includes agglomerates 6 ofelectrically conductive nanoparticles 7. As a result, an anisotropicconnection is produced between the individual diodes by the layer ofadhesive 2, the thickness d of which is less than one micrometer.

A flat conductor is arranged on the underside of the bottom diode, as acommon cathode 38 of the diode cascade of this electronic module 1 andis modeled into a pin protruding from the diode cascade. In acorresponding way, a common anode 39 is placed on the top side 41 of thediode cascade and likewise structured so as to protrude to form a pin.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. An electronic module comprising: metallic surfaces of components ofthe module arranged facing one another; an electrically conductive layerof adhesive between the metallic surfaces, the adhesive includingagglomerates of electrically conductive nanoparticles, surrounded by anadhesive base composition, wherein the nanoparticles have a meandiameter of between 10 and 200 nanometers; wherein the electricallyconductive nanoparticles comprise electrically conductive paths in theadhesive base composition; and wherein the metallic surfaces that faceone another are electrically connected in a punctiform manner via amultiplicity of agglomerates of electrically conductive nanoparticles,which are distributed randomly in the layer of adhesive.
 2. Theelectronic module of claim 1, wherein the layer of adhesive electricallyconnects, without any short-circuiting, a plurality of metallicsurfaces, which are arranged next to one another and are ofmicroscopically small dimensions, to metallic surfaces arranged facingthem.
 3. The electronic module of claim 1, wherein the electricallyconductive nanoparticles include one of the group comprising gold,silver, copper, nickel, and alloys thereof.
 4. The electronic module ofclaim 1, wherein the layer of adhesive has a thickness of less than 1micrometer.
 5. The electronic module of claim 1, wherein the componentshave coefficients of thermal expansion that differ by at most a factorof three.
 6. The electronic module of claim 1, wherein the layer ofadhesive includes a proportion of nanoparticles of between 30 and 95weight %.
 7. The electronic module of claim 1, wherein the adhesiveincludes, as a starting material, a polyamide acetate which is enrichedwith nanoparticles and dissolved in N-methylpyrrolidone.
 8. Theelectronic module of claim 1, wherein the adhesive includes catalystmaterials and adhesion promoters in an adhesive base composition ofpolyamide in addition to the nanoparticles.
 9. The electronic module ofclaim 1, wherein at least one of the metallic surfaces is arranged on asemiconductor chip.
 10. The electronic module of claim 1, wherein atleast one of the metallic surfaces is arranged on a ceramic substrate.11. An electronic module comprising: metallic surfaces of components ofthe module arranged facing one another; an electrically conductive layerof adhesive between the metallic surfaces, the adhesive includingagglomerates of electrically conductive nanoparticles surrounded by anadhesive base composition, wherein the mean diameter of the agglomeratesof electrically conductive nanoparticles in the adhesive is greater thanthe thickness of the layer of adhesive; wherein the electricallyconductive nanoparticles comprise electrically conductive paths in theadhesive base composition; and wherein the metallic surfaces that faceone another are electrically connected in a punctiform manner via amultiplicity of agglomerates of electrically conductive nanoparticles,which are distributed randomly in the layer of adhesive.